This invention relates to a control apparatus for an automatic transmission of an automotive vehicle. More particularly, it relates to a control apparatus which can improve the performance of an automatic transmission when the vehicle is ascending or descending a hill, thereby increasing the safety and comfort of the vehicle.
The gear setting of a conventional automatic transmission for an automotive vehicle is controlled in accordance with the engine load, as indicated by the degree of opening of the throttle valve, and the vehicle speed. While this manner of control is satisfactory under many driving conditions, it can cause problems when the vehicle is travelling on a hill.
For example, when a vehicle is entering a curve in a road on an uphill slope, the driver of the vehicle may decide to let up on the accelerator pedal in order to decrease the vehicle speed. As a result of his doing so, the throttle valve opening will decrease, and this decrease may cause a conventional transmission control apparatus to control the transmission so as to shift up into a gear which is unsuitable for hill climbing, and the vehicle will have difficulty ascending the slope. In addition, the driver experiences an unpleasant sensation when the transmission upshifts contrary to his expectations. When the vehicle is coming out of the same uphill curve in the road and the driver increases the depression of the accelerator pedal in order to again increase the vehicle speed, since the transmission is in too high a gear, the vehicle can not be accelerated as quickly as desired. In this case, the increase in the throttle valve opening when the driver depresses the accelerator pedal as he comes out of the curve may cause the transmission to shift down into a lower gear for acceleration. The downshifting by the transmission produces a sudden change in the torque applied to the drive wheels, and this sudden change greatly decreases the stability of the vehicle.
A conventional transmission control apparatus also produces problems when a vehicle is travelling downhill. The transmission of a vehicle may be in high gear when it starts to enter a downhill slope. Unless the driver manually downshifts the transmission, it will remain in high gear, and the engine will tend to accelerate the vehicle. On a long slope, it is preferable to downshift the transmission to a gear in which the engine is performing engine braking, which refers to a state in which the engine is actually braking the vehicle rather than driving it. However, many drivers of vehicles equipped with automatic transmissions have a tendency not to perform any manual shifting and leave the transmission in the Drive setting under all forward driving conditions. Such drivers rely totally on the brakes to decelerate the vehicle on downhill slopes, but on very long slopes, prolonged and continuous use of the brakes may cause the brakes to overheat and fail.